On June 6, 1944 also known as "D Day", was one of the largest sea / land invasions in human history and started an allied military advance on Germany from Western Europe (Normandy, France). Knowing the Soviets were pushing back from the east, this most likely put Germany at a disadvantage by:
1 ) Fearing defeat, most Germans deserted the army
2 ) Having to bring more troops from the east to fight the allies landing at Normandy
3 ) Creating 2 war fronts on either side of Germany